Remove MEGAGLEST_VERSION for MG_VERSION_H_CONTENTS

I noticed that mk/macos/CMakeLists.txt has special code for extracting the
megaglest version , but its already done in CMakeLists.txt. This removes the
duplicate detection and changes the users of MEGAGLEST_VERSION to
MG_VERSION_H_CONTENTS.
Change library search path on MacOS

When using mac ports GL/gl.h ends up outside the cmake search paths for
libraries and includes. By adding them to mk/macos/CMakeLists.txt they are now
available and make it possible to build g3d_viewer along with the rest of
megaglest. To make this work properly I had to move the inclusion of
mk/macos/CMakeLists.txt further up the master CMakeLists.txt so merged both IF
APPLE tests in to one.

This has built successfully through xcode and gnumake.
Remove unneeded comments

These comments indicated TODO items and are no longer needed in the final change.
Revert change to quoting

Accidentally included a change of quoting which isn't related to the diff at
hand; reverted with this commit.
Remove duplicate DCUSTOM_DATA_INSTALL_PATH

Within 5 lines CUSTOM_DATA_INSTALL_PATH is defined twice resulting in lots of
compiler warnings; eg

[ 11%] Building CXX object source/shared_lib/sources/streflop/CMakeFiles/streflop.dir/libm/flt-32/s_atanf.cpp.o
In file included from <built-in>:175:
<command line>:7:9: warning: 'CUSTOM_DATA_INSTALL_PATH' macro redefined
        ^
<command line>:2:9: note: previous definition is here
        ^

This removes the second definition.

Conflicts:
	CMakeLists.txt
Merge pull request #39 from goetzk/change-library-search-paths-on-osx

Change library search paths on osx
Merge pull request #40 from goetzk/remove-mac-specific-version-detection-code

Remove mac specific version detection code
Merge pull request #41 from goetzk/remove-duplicate-custom-data-install-path-definition

Remove duplicate DCUSTOM_DATA_INSTALL_PATH
- update miniz code to 1.15
Set wxWidgets_CONFIG_EXECUTABLE path

For some reason wxWidgets_CONFIG_EXECUTABLE isn't being found by cmake while
building. I'm hard coding it in to place to alow the build to progress.
Set CMAKE_INSTALL_PREFIX to empty

This empty value means paths will work correctly as relative paths when
building locally.
This change cannot be pushed.
Remove CMAKE_INSTALL_PREFIX definition

This was added while I was flailing around at the start and according to a test
build i've just done is no longer needed.
Remove hard coded wxWidgets_CONFIG_EXECUTABLE

I've spent the time to figure this out using the users environment so this can be removed.